In CTU, which statement is true?

In CTU, which statement is true?

• A. The Accumulated value increases by one
• B. The Preset Value increases by one
• C. The Done bit is set when ACC is greater than or equal to the Preset
• D. The Accumulated value resets on new scan

Answer: (C)

Count Up (CTU) counts how many times a condition occurs, using an accumulator (ACC) and a fixed preset (PRE). Each time the rung condition is true, the ACC increases by one. The PRE value stays constant and does not change by itself. The DONE bit goes high when the accumulated value has reached or passed the preset, i.e., ACC >= PRE. This is why the statement about the DONE bit being set when ACC is greater than or equal to the Preset is the correct one. The accumulated value does not reset automatically on a new scan unless a separate reset action is used, so it can carry its value across scans until you reset it. The other points either miss that the preset is fixed, or that the DONE bit is tied to reaching or exceeding the preset, or that resets aren’t automatic with each scan.

Count Up (CTU) counts how many times a condition occurs, using an accumulator (ACC) and a fixed preset (PRE). Each time the rung condition is true, the ACC increases by one. The PRE value stays constant and does not change by itself. The DONE bit goes high when the accumulated value has reached or passed the preset, i.e., ACC >= PRE. This is why the statement about the DONE bit being set when ACC is greater than or equal to the Preset is the correct one. The accumulated value does not reset automatically on a new scan unless a separate reset action is used, so it can carry its value across scans until you reset it. The other points either miss that the preset is fixed, or that the DONE bit is tied to reaching or exceeding the preset, or that resets aren’t automatic with each scan.